Propose removing ProcessPlanningRequest

[Levi-Armstrong]

@mpowelson and I were talking offline in response to PR #213 and there is duplication between this data structure and ProcessPlanningProblem. The current reason ProcessPlanningRequest exist is because the planning server maintains a cache of environments. Though if this was removed and left up to the user to maintain this data structure could be removed and the planner server would only accept ProcessPlanningProblem. This would also enable the user to submit problem with complete different environments. The responsibility of maintaining the cache would be moved to the ROS planning server instead of restricting the low level planning server to a single environment. @marip8 and @marrts any thoughts?